1. Technical Field
The present invention relates to a projecting zoom lens system for a projector apparatus that projects and magnifies a light bulb-displayed image onto a screen and, more particularly, to a projecting zoom lens system suitable for a liquid crystal color projector apparatus.
2. Description of the Related Art
FIG. 19 shows one of typical optical system of a liquid crystal projector. A projector 8 capable of magnifying and projecting an image onto a screen or the like normally includes a projecting zoom lens system 1 for projecting an image supplied thereto from the incidence side onto a screen, and an image-forming device 7 for supplying an image to the projecting zoom lens system 1. In the case of liquid crystal projectors, liquid crystal panels are used as light bulbs of the image-forming device 7. The projector 8 shown in FIG. 19 is equipped with a white light source 6, dichroic mirrors 5R and 5G for color separation of light radiated from the light source 6, and three liquid crystal panels 3R, 3G and 3B. These liquid crystal panels are transmission display media and each is act for forming an image of the respective separated color, such as red, green and blue. The images for projection formed by the liquid crystal panels 3B, 3G and 3R are directed to a dichroic prism 2 by reflecting mirrors 4. After the individual color images are combined, the combined image falls on the projecting zoom lens system 1. Thus, the images displayed by the liquid crystal light bulbs 3B, 3G and 3R are combined and magnified, and then imaged on a screen 9.
The projecting zoom lens for a liquid crystal projector needs to have a long back focal distance to allow place the dichroic prism 2. Furthermore, because dichroic prisms have the greatly dependence on angle of incidence due to their spectral characteristics, the optical arrangement of liquid crystal panels must be telecentric system to the dichroic prism. In addition, in the case where crystal light bulbs are used, not limited to the above described projector having three crystal plate for forming an image, due to the relatively small view angles of liquid crystal panels, the image quality highly depends on the angle. Therefore, it is desirable that the incidence side of the projecting lens is telecentric.
Further, there are various demands regarding the performance of the projecting lens. In order to produce brighter and larger images at a shorter projecting distance while using a smaller-size projector apparatus, an increase in the angle at the wide-angle extremity of zoom and a reduction in the diameter ratio of lens system are demanded. To effectively project images formed by the liquid crystal panels having a higher pixel density, a higher performance projecting lens is being desired, in which the resolution is improved; the distortion and the chromatic aberration of magnification are sufficiently corrected; the brightness is increased; and the f-number is reduced.
However, in an optical system where a portion on a side toward the liquid crystal panels, that is, an incidence-side portion, is telecentric, rays from a central portion of a liquid crystal panel are imaged on a screen through a central portion of the lens while rays from a peripheral portion of the liquid crystal panel are imaged on the screen through a peripheral portion of the lens. That is, rays from different portions of the liquid crystal panel are imaged through different portions of a lens surface that have different refractive powers. Therefore, in the projecting lens whose incidence side is telecentric, increases in astigmatism, curvature of field and chromatic aberration of magnification, as well as occurrence of a distortion aberration, which tend to adversely affect the imaging performance to a great extent.
Japanese patent application laid-open No. Hei 7-218837 discloses a projecting zoom lens employing three groups of lenses: first to three lens groups having positive, negative and positive refractive powers, respectively, and arranged sequentially in that order from the screen side. In the projecting zoom lens of this construction, the first lens group, disposed on the screen side, essentially has a larger outside diameter than the other lens groups, resulting that the ratio of diameter of lens system becomes large. Therefore, this projecting zoom lens construction is unsuitable to increase the aperture and/or diameter of the entire projecting zoom lens and, therefore, does not allow a significant reduction in the f-number.
Accordingly, it is an object of the present invention to provide a projecting zoom lens system whose incidence side is telecentric, wherein a wider angle is made possible and a compact construction with a large aperture is achieved by reducing the diameter ratio of the lens system. Another object of the present invention is to provide a projecting zoom lens having a high imaging ability, with correction of aberrations well accomplished. It is still another object of the present invention to provide a projecting zoom lens system wherein aberrations are well corrected at the wide-angle extremity and the telescopic extremity and over the intermediate range therebetween. So that a projecting zoom lens system having an increased resolution, a reduced distortion and a reduced color aberration are achieved. A further object of the present invention is to provide a projector apparatus that is able to produce a large-size and bright image at a short projecting distance while being compact in construction, by using the aforementioned projecting zoom lens system having high imaging ability.